In mammals, two tissues are primarily responsible for adaptive thermogenesis: brown fat and beige fat. Although brown fat has long been known as a specialized thermogenic tissue, beige adipocytes, a morphologically distinct type of fat cells that develop within white fat depots upon exposure to cold or high-fat diet, have been discovered only recently. Similar to brown adipocytes, beige adipocytes contain abundant mitochondria and seem to express mitochondrial uncoupling protein 1 (UCP1), which promotes mitochondrial production of heat by increasing the passive proton (H+) leak of the inner mitochondrial membrane. However, the mechanisms of mitochondrial uncoupling and thermogenesis in various beige fat depots as well as the relative contribution of UCP1 remain unclear. Moreover, brown and beige adipocytes retain partial thermogenic capacity even in UCP1-deficient mice. Therefore, this proposal is focused on the identification and characterization of UCP1-dependent and UCP1-independent mechanisms of thermogenesis in brown and beige fat. To this end, we developed a technique for direct patch-clamp recording from vesicles of the whole intact inner mitochondrial membrane (mitoplasts), which for the first time allowed high-resolution functional analysis of mitochondrial ion channels and transporters in their native membrane environment. Using this method, we have succeeded in recording thermogenic UCP1-dependent H+ leak currents across the inner mitochondrial membrane of brown and beige fat. Interestingly, not all beige fat depots possess this UCP1- dependent mitochondrial H+ leak, and using UCP1-deficient mice we also have discovered a novel UCP1- independent mechanism of mitochondrial uncoupling and thermogenesis in both brown and beige adipocytes. This proposal has a single specific aim: to characterize the mechanisms of mitochondrial uncoupling and thermogenesis in adipose tissues. Because adaptive thermogenesis in brown and beige fat consumes large amounts of energy from fat depots and reduces body adiposity and weight, this project will aid the development of therapeutic interventions to control obesity and diabetes.